大阪大学　核物理研究センター村松　憲仁

RCNP workshop for J-PARC, 11 Nov. 2007 @ Osaka

低エネルギー K+ ビームを用いた物理

~~~ Contents ~~~- LD2 results at Spring-8/LEPS- Objectives of + search at J-PARC- Considerations on experimental setup

~420 MeV/c K+n +

Quasi-free photoproduction of + at Spring-8/LEPS

• K+ and K- were detected by forward spectrometer and ＋ was identified by K+n invariant mass with missing mass technique.

• Target : Plastic counter at 1st publication

Further data collection w/ LD2

#neutron×#photons ~ 3×short LH2 runs

n

K-

+K+

np

n+K-→nK+K-

Θ ＋ from STC

LH2

LEPS 1st publication

• Both (1520) and + peaks were observed after excluding large contribution from photoproduction.

• Significance based on Gaussian + linear BG fit ~5• Preliminary ~ 0.01 b/sr by assuming a constant matrix element.

Results from LD2 runs

|pmin| < 70 MeV/c

LD2 preliminary LD2 preliminary

＋ |pmin| < 70 MeV/c

M(nK+) GeV/c2M(pK-) GeV/c2

(1520)

KKN

K*

N

coherent

Comparison with CLAS• CLAS upper limit (95% CL) : (n+K-)<3 nb

LEPS cross section is larger than CLAS limit more than 1 order.• CLAS spectrometer has toroidal magnet, which have no sensitivity

for negative charged particles in extremely forward direction.• Angle dependence may explain the difference b/w LEPS & CLAS.

nosignal

CLAS G10 (dK+nK-p)

- + was identified by pK- missing mass from deuteron. (No Fermi correction)

- S/N ratio gets higher by tightening a cut to select (1520) photoproduction.

K*

KKp

extra events

p

+ associated with (1520) photoproduction

10 MeVn: 5.5w: 4.2

20 MeVn: 5.0w: 3.5

50 MeVn: 6.4w:4.4

100 MeVn: 5.2w: 3.8

pn

＋

K －

p

(1520)d

• Preliminary differential cross section in 10< d-CMS (K-p)<35 : d/d(d+*) ~ 8 nb/sr by assuming constant matrix element & Titov’s matrix element. (Their difference is an order of 20%.)• CLAS upper limit : (d+K-p)<300 pb• Comments : They omitted * production events. There is no sensitivity for forward going *. There is no contradiction with LEPS. Also there seems to be angle dependence in + photoproduction.

Differential Cross Section

Titov’s cross section(E = 2.0 GeV & spin-parity: 3/2‐)calculated based on (NK*)x (+KN) x deuteron form factor

0<gdCMS(K-p)<10

35<gdCMS(K-p)<90

10<gdCMS(K-p)<35

Experimental Situation of + Search

+ is not established yet. But LEPS data suggests its existence.• Width/spin/parity is not determined.• Affected by reaction mechanism ? - Angle dependence in + photoproduction ex. Comparison of LEPS and CLAS - Energy dependence in + production ex. (+)/(*) < 2-3 % in high energy experiments - Isospin asymmetry in NK+

ex. CLAS proton data killed this reaction. No K* exchange in t-channel cf. E559 (No observation of K+p ++)

Objectives of + search at J-PARC

• Need + formation experiment by K+n resonance - Direct confirmation of + existence - Independent from reaction mechanism - Width can be measured from cross section

- Spin measurement by decay angular distribution 1 for 1/2, 1+3cos2 for 3/2, 1-2cos2+5cos4 for 5/2 Possibility to interfere with BG : odd power of cosinecf. Parity: pp++ or spin-observables in photoproductioncf. Systematic studies of reaction mechanisms at LEPS2 - + photoproduction with high intensity photon beam (~107/sec) and 4 acceptance detector

dEME

Jk 4/)(

)12(8 22

2

2

K+n Scattering Experiments

mpKs (GeV/c2)

N /

2 M

eV/c

2

Nch

DIANA

BelleK+ is ‘reconstructed’ from thereaction D*-D0-(K+-)-

DIANAOld bubble chamber experiment

K+Xe KS0pX

Need a modern experiment with high intensity K+ beam at J-PARC

Basic Concepts

π ＋

π －

proton K+ beam K0.8 ?

ΔP/P=1.4%

417 MeV/c

• Resonance formation reaction:

K ＋ n→+→KS0p→ ＋－ p

　　　‐ P(K ＋ )=417 (442) MeV/c

for M=1.53 (1.54) GeV/c2

+, -, & proton detection

by 4 spectrometer

M(+-)=M(KS0) M(K⇒ S

0p)=M(+)

• Originally considered at BNL-E949 Similar but optimized experiment is possible at J-PARC.

K0.8 (Sharing w/ stopped K+ exp.)

targetK0.8

Fitch-typeCherenkov

BeO degrader ~40 cm(high dens. and low A)

Beam WireChamber

AerogelCherenkov

K+: ~420 MeV/c (~0.648nth~1.54)+: ~600 MeV/c (~0.974nth~1.03)

E787 Year 95 96 97-98 P(K+) MeV/c 790 730 710Stopping Fraction 20% 25% 28%

K

• ~40 cm BeO degrades K+ momentum from 800 MeV/c to 420 MeV/c.

• Lower momentum beam decrease thickness of BeO.

• K/ separation by Cherenkov detectors

Compact beamline elements

(c=3.713 m for K+)• Nuclear interactions at BeO can be

omitted by detecting multi-particles at Beam Wire chamber.

Active Target

proton: vs. P in Lab. sys.

proton

R vs. P in CH- K+ travels inside a target until momentum becomes appropriate to produce +.- Proton is emitted in forward directions, and tends to stop inside the target.- Kinetic energy and polar angle measure- ments of proton.- Mometum correction for pions. Active target w/ fine segmentation

LD2 Target (Inactive)

• smaller dE/dx smaller mom. corr.• Less nuclear interactions for decay

products.• Redundant measurements for

forward going proton

ex. Track direction, TOF, range,

kinetic energy, dEdx meas.

by DC and range conter.

Or large aperture dipole?• Adjustments of K+ beam momentum

may be necessary.

solenoid

LD2 target

DCrange

counter

K+

proton

+

-

Spectrometer Considerations

Pions are detected at spectrometer& proton stops inside the target

cos(polar angle of KS0) of

generated + eventspion: vs. P in Lab. sys.

- Pions are emitted in side directions. Cylindrical drift chamber inside a solenoid.- In case that a 1 m-long drift chamber is placed at -40 cm to 60 cm of the target, geometrical acceptance is an order of 40%.- PID by TOF would be enough. (See right figure: green R=50 cm, red R=90 cm in case of charged particles are emitted at 90. t=50 psec is assumed.)

TPC option

• If only small solenoid is

available, Time Projection

Chamber may be usable.

(PID by dE/dx instead of TOF)• Dead time may be problematic

for high trigger rate. dE

/dx

[arb

itra

ry u

nit

]

Momentum×charge [GeV/c]

PID by LEPS 1st TPC

p

Mass Resolution

w/o kinematic fit

w/ kinematic fit

• Mass resolution studies were done assuming BNL-E949 detector resolutions. Invariant mass of p : 7.6 MeV/c2

(P/P=1.4% at P=200-300 MeV/c, E/E=8.3% at EKIN=100 MeV, proton angle mes. error = 6) Kinematic fit (using correlation with KS

0 mass) : 6.2 MeV/c2

• ~1% resolution is desired for a cylindrical spectrometer.

Expected Yield

• In case of BNL : 1012 proton/pulse(=duty cycle) 3105 K+/pulse @710 MeV/c 3104 K+/pulse @475 MeV/c w/o degrader• Y= l NA FK fn = 1.032 g/cm3 25 cm 10-27 cm2

6.0221023 3104 /pulse (6/13) = 200 /mb/pulse This yield will be reduced by detector acceptance, nuclear interactions, and so on. BW(E) = /(4k2) 2/[(E-M)2+2/4] for spin1/2 26.4 mb/MeV• K-p→(1520)→ ＋－ for calibrations and checks of data

quality and analysis procedure with the same beamline and detectors. (It is worth to do even if K- intensity is a bit lower.) : (*)=15.6 MeV order of 100 mb

CEX background

Main BG contribution comes from CEX (K ＋n→KS0p).

total cross section ~7 mb [PRD15(1977)1846]

Summary

• Although LEPS LD2 data shows clear peak of +, its photoproduction seems to be affected by reaction mechanisms. LEPS2 with higher intensity beam & 4 spectrometer.• Existence of + can be directly confirmed in K+n resonance reaction at J-PARC by using ~420 MeV/c high intensity K+ beam. Measurements of width & spin are possible.• LOI Proposal in several months• Beamline and detector system can be shared with rare kaon decay experiments.